1. Field of the Invention
This invention relates to an optical system for an endoscope for providing divergent rays of light from an illumination light guide so as to illuminate a wide field of view of the endoscope.
2. Description of Related Art
Typically, endoscopes or fiberscopes have an image guide fiber bundle and a light guide fiber bundle both of which are in a flexible, elongated sheath or tube. In order to view or photograph an object through the image guide fiber bundle, the endoscope handled with the forward end of the light guide fiber bundle directed closely to the object so as to illuminate the object.
One recent design demand for such an endoscope, in particular a medical endoscope, is to reduce the outer diameter of a flexible sheath or tube as small as possible for relieving or not causing pain to the patients while providing ease of observation. In order for the endoscope to satisfy this demand, it is essential to install both the image fiber bundle and the light guide fiber bundle in as thin and compact package as possible. This unavoidably requires the installation of an objective lens having a wide field of view along with an illumination optical system having a wide angle of illumination. Japanese Unexamined Utility Model Publication No. 2-140519 reveals one technique for widening an angle of illumination of the illumination optical system of the endoscope. As shown in FIG. 2, a wide angle illumination optical system, as described in the above mentioned publication, includes a plurality of lenses, such as a convex lens 12A, a convex lens 12B and a concave lens 13 arranged in order from the fiber bundle side. The first two convex lenses 12A and 12B forms a convergent lens system functioning to gather illumination light emanating from the light guide fiber bundle 11 and focus it at a point. The concave lens 13, which is a divergent lens, functions to diverge the illumination light.
However, to provide the prior art illumination optical system with a sufficiently wide divergent angle covering a sufficiently wide field of view of an objective lens practically requires more than two convex lenses. Any increase in the number of optical elements included necessary results in an increase in manufacturing cost of the endoscope, and such is undesired.